1. Field of the Invention
This invention relates to a bar code reader for two-dimensional bar code symbologies. More particularly this invention relates to an improvement for positioning a two-dimensional bar code scanner in alignment with the symbol to be read such that the scanner's optics are at an optimum distance therefrom.
2. Description of the Prior Art
Two dimensional bar code symbols are now common in the art. Symbologies such as PDF417 are characterized by multiple rows of data that in aggregate contain encoded information. Usually redundant information is included for error detection and correction. PDF417 is described in Wang, U.S. Pat. No. 5,243,655. Other two-dimensional symbologies are known. For example, Allais, U.S. Pat. No. 4,794,239 discusses stacked linear bar codes, and proposes a multi track symbology.
Known bar code reading devices are capable of reading symbols even when the scanning axis of the reader is not rotationally aligned with the rows and columns. In the case of PDF417, this can be accomplished by "scan stitching", which utilizes positional information that is encoded in each row to identify a given row being scanned. Another approach, described in European Patent Application EP 0 385 478, involves transferring the optical pattern into a memory, and then evaluating the contents of the memory to determine the angular orientation of the symbol. Nevertheless it is more efficient to read the symbol in alignment with the active optical elements in the scanning device.
It is desirable to operate portable bar code scanners with light sources that consume low power, such as light emitting diodes. These devices have carefully optimized optical systems for creating an illuminating beam with sufficient angular divergence to read a bar code symbol that is wider than the optical window of the scanner, yet limit off-angle falloff of illumination to acceptable limits. Such scanners cannot be positioned too far from the symbol and still read effectively. Also, they often do not tolerate deviation of the scanner's optical axis from a normal to the surface of the target, because the working depth of field is intentionally kept small, in order to take advantage of the limited illumination available.
Sakai, U.S. Pat. No. 4,210,802, discloses a scanner in which two parallel guide rails appended to the front of the housing are proposed to define a scanning region for the image sensor. Parallel top and bottom walls are provided to enable the operator to visualize the symbol. This arrangement could assists the reading of linear bar code symbols, but would be inconvenient for use with a two-dimensional symbol, as the operator would not have a ready 360.degree. view of the symbol's borders.